The development of modern electronic devices places an ever increasing demand on the quality of the semiconductor materials used in these devices. Commonly used substrates for growing semiconductor materials, such as sapphire and Si, typically cause large defects in the semiconductor materials grown on these substrates, which in turn adversely affect the performance and reliability of the electronic device employing the semiconductor materials.
The poor quality of the epitaxially grown semiconductor crystals is generally due to the unavailability of a lattice matched and chemically matched substrate. When a film is grown on a substrate such that the crystal structure of the substrate is reflected on the film, the thus-grown film exhibits microstructural discontinuity at the interface between layers formed of different semiconductor materials (i.e., the interface between the substrate and the film grown on the substrate). Since the thickness of the film is considerably smaller than that of the substrate, the difference in lattice constant between the film and the substrate causes generation of strain and defects in the film.
For example, when a crystal of a nitride semiconductor, such as gallium nitride (GaN), is grown on a starting substrate made of silicon (Si) or the like and then cooled to the ordinary temperature, a large number of dislocations and cracks occur in the nitride semiconductor layer because of stress caused by the difference in thermal expansion coefficient or lattice constant. If a large number of dislocations and cracks occur in the growth layer (nitride semiconductor layer), lattice defects or a large number of dislocations, deformations, cracks, etc., occur in a film when the film is epitaxially formed on the growth layer. This causes deterioration of the film characteristic.
Certain oxide substrates, such as ZnO, show some promise, but they often react with by-products during the epitaxial growth. The best commercially available substrate for growing semiconducting films is SiC. However, SiC wafers, especially large SiC wafers, are very expensive.
Therefore, demand has arisen for the development of a high quality substrate for growing compound semiconductor films with reduced crystal defects.